Method of treating oil well casings



Aug. l1, 1942.

T. J. CRAWFORD METHOD OF TREATING OIL WELL CASTINGS Filed Aug. 18, 1941 --JOOO vINVETOR THOMAS 1 c/PA wforep Patented Aug-1,1, 1,942

UNITED STATE METHOD VThomas J.

to Republic Steel Corporation,

F TREATING OIL WELL CASINGS Crawford, Youngstown, Chio, assignor Cleveland, Ohio,

a corporation of New Jersey 'Application August 1s, 1941, serial Nu.

7 Claims.v (Cl. 148-21) This invention relates to the art-of steel pipe and its manufacture and is particularly concerned with steel casing for use in deep oil wells, and to a new and improved method for making oil well casing which will have improved resistance to collapse and improved joint strength,A

Steel casing for oil wells is subjected to external forces which increase with the depth of the well and which must be resisted to avoid failure of the casing. Such casing is made in pipe lengths which are about 40 feet long and are connected together by couplings threaded to the ends of adjacent lengths of pipe. Since the Weight of the string of casing is carried by these threads and coup ngsy the threaded portions of the pipes must have a high joint strength to prevent parting of the string and to permit removal of the string from the well should the occasion demand. Joints having standard threads usually part because of the radial compression of the threaded ends of the pipe which takes place under4 tension, such distortion allowing the threads to disengage. Joints having acme or square threads usually fail by breaking off the pipe at the thread root beyond the last engaged casing must be sumciently ductile to resist breakage in instances where the drilled hole is not perfectly straight.

One important specification for oil well casing has been that of relatively high ductility. Such high ductility has been accompanied by collapse strengths and joint strengths which `were not as high as were desirable. Various expedients have been tried to increase the collapse efciency and joint strengths of casing without materially decreasing the ductility of the casing, but so far as I know these efforts have not satisfactory. Y

The American Petroleum Institute, appreciating the need in the industry for a. joint of higher strength, yet using standard thread dimensions and couplings, has recently appointed an engineering committee to work on the 'problem of designing such joints on American Petroleum Institute standard 5%" and 7" casing where rotary drillv pipe is not used inside the casing except in emergencies. One proposal is to upset the threaded ends of the pipe internally to provide suicient metal under the threads to give the desired strength.

By the present invention I am able to increase joint strengths oi casing not only of those sizes but also of larger sizes within which drill pipe is used for drilling and accomplish these results without the extra cost, reduced clearances and localized wear which are inherent factors in upset pipe ends By the present invention I am able to increase the collapse strengthof casing as much as or more and to increase the joint strength as much as or more, without materially decreasing th`e ductility of the casing. Briey stated, I increase the collapse strength and eiliciency of the casing in any one of several ways. I may quickly heat a thin interior layer of the casing to a temperature between about 500 F.

thread. Furthermore, cil well and the temperature at toredissolve, which forlsteels suitable for casing is about 1300 F., and then cool the pipe; or I may quickly heat a thin interior layer to above the AC3 temperature of the steel and quench it and, if and when necessary to improve ductility, I may temper or draw the resulting hardened layer by quickly reheating it to a temperature between about 500 F. and about l300 F., and cool the pipe. Where it is desired to preserve the effects of previous cold working of the pipe in the portions thereof which havenot been hardened by heat treatment, the drawing operation should be carried out rapidly and locally so that the heating effect is confined to the hardened material. When the drawing operation undesirably changes stresses in the pipe and it is desirable to improve the temperature between about 500-F. and about been entirely ciencies of casing and adjacent to the pipe 1300 F., and then cool the pipe.

I increase the joint strength by quickly heating a thin interior layer of the casing at and ends to a temperature above the Aeg temperature of the steel and then quenching and hardening this heated layer.

In the drawing accompanying and forming a part of this specication,

Figure 1 is a perspective view, tion, of a length of oil well casing embodying the present invention; and

Figure 2 shows curves illustrating collapse empresent invention.

In Fig. l, I designates a length of oil well casing, 2 designates the exterior threads at each end thereof which are to engage with couplings (not shown), and 3 indicates the thin hardened layer of metal on the inside of 'the casing within the threaded portions. In general, the ratio of the diameter of the casing to the wall thickness, i. e.,

t. f ranges from about 12 upwardly, and the present ,invention insofar as improving collapse strength is concerned is directed to steel casing having a t ratio of more than about 14.

which the carbon begins) stresses, I may quickly and locally heat the exterior of the pipe to a partly in secthe effects thereon of the An electric 'of electrical power which is ratio of 14 or more is ail'ected by the presence of stresses which, for lack of a better description, have been called If a short length or ring of casing having such stresses is cut longitudinally, i. e., axially, the casing tends to spring open, thereby opening the cut,- when the tangential residual stresses are positive, and tends to spring inwardly, thereby closing the out, when these stresses are negative. I have also found that when a length of casing of homogenous structure possesses substantially no tangential radial stresses its collapse strength and emciency is at the maximum, and that as thesestresses increase in magnitude. whether they are positive or negative, the collapse strength and eillciency decreases.

Steel casing having a ratio above 14 as ordinarily made heretofore, usuallyas a result oi' cold rotary straightening-possessed tangential residual stresses which were positive and which varied in amounts froml a few thousand pounds per square inch to 20,000

pounds per square inch or` more. These stresses can be diminished and in some cases reversed from positive to negative values by the treatment above described. As these stresses approach a minimum, the collapse strength and eillciency increase.

I have found that when a thin surface layer of a steel pipe is heated to between about 500 F. and the temperature at which the carbon begins to redissolve, and when such heating is accomplished within a proper short time interval, and when the thus heated pipe is then cooled, the undesired tangential residual stresses will be relieved largely or wholly. V'

One convenient and satisfactory method of treating casing such as isshown at i in Fig. 1 to improve its collapse efllciency, is as follows: induction heating element in the form of a s hort cylindrical member slightly less in diameter than the casing to be treated is inserted in the casing and is connected to a source suillcient to heat the inner surface of the pipe by induction to a temperature between about 500 F. and about 1300 F., within a very short time, for example within a few seconds. Relative axial movement of the heater and pipe is created, and preferably the heater alone is moved. The rate 'of this relative movement may range from about 1%" upwardly to 15" or more per second, depending upon the power input available. It will be understood that the speed of such relative movement may be varied depending on the power input, but the speed and power input should be so regulated that only the inner surface or a very thin inner surface layer is heated to the desired temperature, and that this heating is done very rapidly and, in effect, locally for it is important that the casing should not be heated throughout its wall thickness to the peak temperature but that the heat supplied tothe thin inner layer should dissipate itself by spreading to the remainder of the casing' wall. Preferably the pipe is rotated during relative axial movement of the pipe and heater to overcomethe effects of variations in clearance therebetween.

The following conditions maybe'stated as being suitable for relieving, wholly or in part. posi- ,tangential residual stresses."

. temperature of each increment oi' the i pounds per square inch and a tive tangential residual stresses in a length of steel casing Awhich is 5" in outside diameter and hasv a wall. When an induction heater about 2" long and having an outside diameter slightly less than the inside diameter of the casing is moved through the casing at the rate of about 1%" per second, a thin inner surface layer of each annular increment of the casing will be heated in about 1.6 seconds to a temperature of about 600 F. when a current of about 340 volts and about 170 amperes 'at 3000 cycles is applied thereto. A similar inner surface layer will be heated to about 800 F. when the current applied to such a heater in such a pipe is about 455 volts at about 230 amperes and about 3000 cycles. Similarly, such a thin inner surface layer will be heated in the same length of time to about 1300* F. when-the current supplied to the heater is about 600 volts at about 300 amperes and 3000 cycles.

When thestrength of the pipe is to be increased, as discussed herein, by heating to between about 1600a F. and about 1800 F. and quenching. a current of about 800 volts and about 430 amperes and 3000 cycles will raise the thin annular surface layers to about 1700 F. in about 1.6 seconds using the induction heater above described.

When a having a tistical minimum values on which design factors of safety are based will be raised still more in proportion. A

Curve l of Fig. 2 illustrates not only the relationship existing between collapse emciency and transverse residual stresses in steel casing havinga t ratio of 14er above, but also the improvements in collapse efficiency which I have obtained by means of Vthe above described` treatment. Lengths of casing which have been straightened while cold by the rotary methodhave positive tangential residual stresses of various amounts usually greater than about 5000 lbs. per square inch and are illustrated by the upper part of curve `5. When a plurality of such casing lengths is treated as above described, those stresses are decreased and may be reversed with the result that the group of points representing the stresses in the various. casings is moved closer to the node of the curve and the average and the minimum collapse efliciencies are increased. As curve 5 indicates, ciency of a length of such casing is about 100% substantially no transverse residual stresses are present.

The point A on curve l indicates a similar length of casing which, before being treated as above described, had a positive transverse residual stress oi' about 7000 pounds per square inch and a collapse eillciency of about Point B illustrates a length of casing which had a positive tangential residual stress of about 9400 collapse emciency of about 92%. Point C on curve i illustrates anplurality of lengths of steel casing that layer.

other length of similar casing after it had been treated, as described above. It had a negative tangential residual stress of about 3000 lbs, per sq. in. and a collapse efliciency of about 99%. is believed that the specimen indicated by point C had a low positive transverse residual stress before treatment and that the treatment reduced that stress thru zero and on to the indicated negative stress. These specimens show how itis possible, by this invention, to improve the collapse strength and efficiency of cold-rotarystraightened casing.

The illustrative figures were obtained on homogenous casing, i. e., a pipe which is substantially uniform in structure thruout its wall section. These results were obtained with steel containing between about .20% and about .24% of carbon and-between about .61% and about .65% of manganese.

Instead of using the low temperature treatment described above, I may use a higher temperature and harden an inner thin layer of the casing. As briey described above, this treatment consists of quickly heating a thin inner layer of the casing to above of the steel and then quenching and hardening Curve B of Figure 2 illustrates improvements in collap'se strength and eiiiciency which have been obtained by such a high temperature treatment. The specimen illustrated by E on curve 6 had a positive tangential residual stress of about 22,000 pounds per square inch with an accompanying collapse eiiiciency of about 125%. Similarly, another specimen illustrated by D had a positive transverse residual stress of about 15,000 pounds per square inch and a collapse efficiency of about 127%. F represents another specimen whose positive transverse residual stress was about 9400 pounds per square inch and whose collapse eiciency was about 120%.

When casing is provided with such a hardened inner layer its ductility is decreased somewhat, The ductility may be improved by reheating the hardened layer quickly and, in effect, locally as previously described, i. e., by heating it rapidly to between abouti-300 F. and about 1300 F., and cooling it. Point G on curve 6 illustrates casing similar to the casing of points D and E but drawn as just described. This specimen had a collapse eiciency o f about 88%,4 a negative tangential stress of about 28,320 lbs. per square inch and increased ductility (not shown by the curve) as compared with ing treatment.

The collapse eiiiciency of casing such as is illustrated by the hardened and drawn specimen of point G on curve 6 may be increased by subjecting the exterior surface to a low temperature treatment, for example, by quickly heating the outer surface to a temperature between about 500 F. and about 1300 F. and cooling it. Such a' treatment will decrease the negative transverse residual stress and increase the collapse efficiency.

The joint strength may be lincreased in the following manner: A heating element'as above described is inserted within the end of the pipe to be threaded and suiiicient electrical power is employed to heat the inner surface layer of the casing to a depth of, for example, .075" to a temperature above the Aca point of the steel, for example, to between about 1600 F. and about 1800 F. for steel of the foregoing composition, and to do this heating within a very short period the Aca temperature its ductility before this drawmoving the heater of time, for example only a few seconds. The heater may remain stationary if it is of sumcient length to heat the desired length of the casing at and adjacent to the part to be threaded, or, if the heater is not that long the heater and casing may be moved relatively at a speed which may range upwardly to about 15" per second or more, depending upon the power input. Immediately after thesurface of the casing has been heated to the desired temperature it is quenched, as by spraying water on it. This quenching `may be accomplished by first removing the heater or by in the pipe and following it with a quenching spray,

The eiect of this treatment is to increase the hardness of the highly heated and quenched inner layer. For example, I have obtained inner hardened layers having hardnesses of from about 30 to about 46 on the Rockwell C scale, while the hardness of the remainder of the pipe wall was about on the Rockwell B scale. The untreated pipes failed in pullout tests under loads of about 175,000 pounds, while corresponding pipes treated as just described failed at pullout loads averaging about 315,000 pounds. Thus pipes so treated had strengths which were increased by about 80%.

The heating and quenching treatment somewhat reduced the ductility of the pipes at their ends but this decrease was not serious because of the support afforded by the coupling. However, the ductility of pipes thus heat treated and quenched at their ends may be increased by a drawing treatment, i. e., by rapidly heating the thin hardened layers to between about v500 F. and about 1300 F. In other words, the iirst collapse improving treatment described above may be carried out on the heat treated and hardened pipe ends with the result that the ductility will be considerably increased without much decrease in joint strength. For example, pipes having heat treated and quenched ends as above noted failed in a pullout test at about 315,000 pounds, which was about 180% of the strength of untreated pipes. After being treated by this drawing operation the pullout failure strength dropped to about 287,000 pounds, which was still about %V of the corresponding strength in untreated pipes.

When a length of casing vis subjected to the high temperature and hardening treatment the improvement in joint strength will be obtained simultaneously with the improved collapse efflciency. When the hardened layer is to be drawn to improve ductility, the drawing treatment is preferably not applied are to be threaded.

From what has been said hereinabove, it will be understod thateither the collapse improving treatment or` the joint strength improving treatment lmay be carried out together or independently of one another. For example, if it is desired to increase both the collapse and joint strengths of a length of casing, the heater may be passed axially thru the length of pipe' and the heat may be so adjusted as to raise the temperature at one end to be threaded to above the Ac: point and then decreased to heat the portion of the pipe between they ends to be threaded to a temperature in between about 500 F. and about 1300 F. and then the heat may be increased when the heater comes to the other end to be threaded so that it will be heated to at the first end. When this method is employed, the highly heated inner to the end portions which surface of each end is quenched` immediately after the heating. This may conveniently be done by embodying in the heater a water-'carrying tube thru which water may be sprayed onto the heated walls immediately baci:` of the moving heater. When this procedure is followed and it is desirable to increase the ductility of the hardened end portions of the pipe, the heater may be re-employed to reheat the quenched portions of the pipe to a drawing temperature, i. e., from about 500 F. to about 1300 F.

It will be understood that it is not essential to use an electric induction heater as described above but that any other .heating means may be employed which is'suitable for; carrying out the present invention.

I believe that one explanation for the results measured in collapse efficiency of pipes treated by the above described method is, that the method makes the pip slightly smaller in diameter than it was before the treatment. It is my belief that when, for example, a thin annular inner surface-layer is heated by this method it expands and increases the elastic hoop tension of the annular layer radially outside thereof, and that as the heating progresses the inner layer reaches a temperature at which it rapidly loses its compression strength, whereupon the stresses previously stored in the cool outer layer are able to relieve themselves by compressing the inner heated layer tangentially. If at this point the heat input is stopped, and the heated layer cools, it soon regains its strength when subsequent shrinkage thereof tends to draw the outer layer inwardly with it, thereby bringing about a final condition of reduced tangential stress in the outer layer or even reversing those stresses to a condition of tangential compression and with an eventual slight' decrease in the diameter of the casing as compared with the diameter be` fore this heat treatment.

This application is a continuation-in-part of my copending application Ser. No. 327,018, filed Mar. 30, 1940, now Patent No. 2,256,455, which application claims a steel pipe having improved strength at the threaded ends thereof.

Having thus described my invention so that others skilled in the art may be able to practice the same, I state that what I desire to secure by Letters Patent is deiined in What is claimed is: 1. The method of treating steel pipe having a what is claimed.

F., and the temperature at which carbon begins to redissolve, and cooling the pipe.

2. The method of treating steel pipe having a i ratio above about 14 which includes the steps of heating a radially thin inner annular surface layer of the pipe to a temperature between about 500 F.. and the temperature at which carbon begins to redissolve, and cooling the pipe.

3. The method of treating steel pipe having a t ratio above about 14 which includes the step of heating axial increments of a radially thin, annular surface layer of the pipe within a few seconds to a temperature between about 500 F.. andu the temperature at which carbon begins to redissolve, and cooling the pipe.

4. The method of treating steel pipe having a ratio above about 14 which includes the steps of progressively heating axial increments of a radially thin, annular surface layer of the pipe in about two seconds to a temperature between about 500" F., and the'y temperature at which carbon begins to redissolve, and cooling the pipe.

5. The method of treating steel pipe having a inga ratio above about 14 which includes thev steps of heating axial increments of radially thin, annular inner surface layers thereof at each end of the pipe to a temperature above the Aca range of the steel within a period of a few seconds for each increment, quenching each of said increments immediately after it attains such temperature, heating axial increments of a radially thin. annular inner surface layer of the pipe between said end layers within a few seconds to a temperature between about 500 and the temperature at which carbon begins toredissolve, and cooling said layer.

7. The method of treating a steel pipe having a ratio above about 14 which includes the steps of progressively heating a radially thin, annular inner surface layer thereof from end to end of the pipe in axially short increments, the layer which is substantially coextensive axially with the threaded end portions of the pipe being heated to above the Ac; range of the steel within a few seconds for .each increment thereof and then quenched, and the layer between sa-id end portions being heated within a few seconds for each increment thereof to a temperature between about 500 and the temperature at which carbon begins to redissolve, and then cooled.

. THOMAS J. CRAWFORD.

6. The method of treating a steel pipe hav- I 

